Wednesday, 19 December 2007

CERN will soon fall into a winter sleep officialy called the annual closure. In this period the body activity is reduced to a minimum. The personel is sent on a forced leave of absence, only a few scattered die-hards remain on site. There is no life as we know it: the cafeteria is closed, heating is switched off and coffee machines are removed for fear of looters. Sadly, this implies there is nothing to blog on. CERN will come back to life January 7; I may reopen before that.

Since always, the TH Christmas play has marked the end of the year at CERN. Typically, the script comments on the important events of the year. The audience loves the show for Pythonesque humor, ruthless satire and explicit content. Previous plays created undeniable stars: Georg Weiglein has been known as Shrek ever since, while Ben Allanach's Borat is legendary, especially among the female part of CERN staff. The late DG, after seeing the play in his honour the other day, ended up convinced that the Theory Group is expendable and cut down on our budget.

This year, the play showed a very inside story of the recent DG election. The story begins with Snow Pauss and Eight Dwarves competing in a beauty contest. Three dwarves get most applause:

"Rolfy" Heuer (played by John Ellis) who promises to give all the CERN geld to the ILC

"Hairy" Ellis (played by no-matter-who, since the beard covers him all) who promises to cut his hair

"Roby" Petronzio, who at this stage appears irrelevant, but who will play the key role in the following

The remaining candidates are downgraded to uncandidates, despite violent protests from Snow Pauss' supporters. The chosen three face each other in a three-man standoff a-la The Good, the Bad and the Ugly. In the original movie, the Good shoots the Bad, while Ugly finds his gun unloaded. This time, however, "Roby" the Ugly shoots "Hairy" the Good at the back, thus proving that real life is different than movies.

Update: Photos now available. Here is the scene with the candidates comparing the sizes of their accelerators:
More photos here and here. The uncensored video recording can be downloaded from here.

Friday, 7 December 2007

This week, CERN TH hosted a workshop entitled Interplay of Collider and Flavour Physics. Flavour physics is concerned with processes involving transitions between different generations of the Standard Model fermions. In recent years, flavour physics has seen considerable experimental progress, prompted mainly by the results from the B-factories. However, in spite of great expectations, these experiments brought nothing but disappointment, despair and loss. In short, all the observed phenomena can be well described by the vulgar flavour structure encoded in the CKM matrix of the Standard Model. This means that new physics, if it exists at the TeV scale, must be highly non-generic; to a good approximation it must be either flavour blind or its flavour structure should be correlated with that of the Standard Model.

90% of the theory talks in the workshop dealt with the MSSM, which is kinda funny given that the MSSM has nothing to say about the origin of flavour. Luckily enough, Christophe Grojean (on behalf of the local tank crew) reviewed another approach to TeV scale physics in which flavour is built in. This approach is commonly referred to as the RS scenario (where RS stands for Raman-Sundrum) and its modern incarnation is pictured above. The fermion mass hierarchies in the Standard Model follow from different localizations of fermions in the fifth dimension. Light fermions are localized close to the UV brane and have little overlap with the Higgs boson who lives close to the IR brane. The third generation (or at least the top quark) lives close to the IR brane, thus interacting more strongly with the Higgs. In this way, the fermion mass hierarchies in the Standard Model can be reproduced with completely anarchic Yukawa couplings between the Higgs and the fermions.

The RS scenario predicts new physics states, for example the Kaluza-Klein modes of gluons, with masses not much larger than TeV, as required by naturalness. If couplings of these KK gluons to the three Standard Model generations were completely random, the KK gluons would mediate flavour violating processes at the rate incompatible with experiment. The situation looks however much better. It turns out that the same mechanism that produces the fermion mass hierarchies also aligns the couplings of the KK gluons in such a way that flavour violating processes are suppressed. Most of the flavour violating processes predicted by the RS scenario are within experimental bounds if the mass of the lightest KK gluon is larger than 4 TeV (the same bound follows from electroweak precision tests). The exception is the parameter $\epsilon_K$ describing CP violation in the Kaon system, that comes out a factor of few too large for a 4 TeV KK gluon. This means that the Yukawa couplings should not be too anarchic in the end and we need additional flavour symmetries...or maybe this is just an accident and the problem will go away in a different incarnation of the RS scenario. Future experiments (not only the LHC, but also super-B-factories, or MEG and PRIME) should give us more hint.

Saturday, 1 December 2007

I wrote there was no interesting seminar last week, but i should mention there was an interesting pre-seminar discussion before the Wednesday Cosmo Coffe. The authors of this article were yelling at the speaker of that seminar. The latter is a member of the Auger collaboration and the former just submitted a comment to ArXiv, in which they put in doubt some of the conclusions obtained by Auger.

The new results from the Pierre Auger Observatory were announced a month ago (see also the post on Backreation). Auger looked at cosmic rays with with ultra-high energies, above $6\cdot 10^{19}$ eV. By theoretical arguments, 90% of such high-energy particles should originate from sources less than 200 Mpc away. This is because of the GZK cut-off -- ultra-high energy particles scatter on the CMB photons, thus losing their energy. Auger claimed to observe a correlation between the arrival direction of the ultra-high energy cosmic rays and the positions of the nearby Active Galactic Nuclei (AGN). This would mean that we finally pinpointed who shoots these tennisball-energy particles at us.

In the aforementioned comment, Igor Tkachev i yego komanda points out that Auger in their statistical analysis did not take into account the $1/r^2$ decrease of the flux with the distance to the observer. The claim is that, once the decrease is taken into account, the AGN hypothesis is disfavored at 99% confidence level. The problem is illustrated on the figure to the right. The crosses mark the positions of the nearby AGN, the color shades indicate the expected flux of the ultra-high energy cosmic rays (if the AGN hypothesis is true) and the circles denote the hits registered by Auger. One can see that most of the ultra-high energy particles arrive from the direction of the Centaur supercluster, while none arrive from the Virgo supercluster. The latter is in fact closer to us, and we would expect at least as many hits from that direction. According to the authors of the comments, the more likely hypothesis is that there is a bright source of the cosmic rays that lies in the direction of the Centaurus supercluster.

It seems that we have to wait for more data to finally resolve the cosmic ray puzzle. In the meantime, here is the local map i found while preparing this post. Just in case you need to find your way home...

There were no exciting seminars last week here at CERN. But, since I promised to resume blogging, here I am with this brief report from the Cosmo Coffee last Wednesday. Cliff Burgess was talking about a new string inflation model that is not around yet and that is not of his making - his friends Conlon-Kallosh-Linde-Quevedo are going to fire it off soon. The inflationary model itself does not violate causality, however. It is realized in this string theory model, which is a variant of KKLT, but it assumes different value of parameters and has a different low-energy phenomenology. The model has a metastable vacuum at large volume of the internal Calabi-Yau space, which arises when stringy loop corrections are taken into account.

The new inflationary model is christened volume inflation, because the volume of the Calabi-Yau is the modulus that assumes the role of the inflaton. Inflation happens when the volume is relatively small. This means that the string scale and the Planck scale are close to each other, which is favorable to obtain large enough density fluctuations. After inflation ends, the volume rolls down to our metastable vacuum where the cosmological constant is small and where the string and Planck scales are separated by several order of magnitudes. The model incorporates a mechanism of converting the energy of the inflaton into radiaton, which helps to avoid the overshooting problem and delivers us from evil of a ten dimensional vacuum.

My problem with this string inflation model is that, as Cliff admitted himself, there is hardly any string inflation here. Inflation operates below the string and compactification scales, so that the 4D field theory approximation applies. At the end of the day, it is just standard inflation in field theory, with some building blocks motivated by currently popular string models. The string origin does not produce any smoking-gun imprint. Nor it gives any insight into the conceptual problems of inflation - the vacuum energy problem and the transplanckian problem.

Well, I know that inflation in field theory works fine, if we turn a blind eye on some conceptual problems and allow for some fine-tuning. So, if you want to impress me, try working on string inflation instead. The only attempt i know of in which the ideas of string theory are crucial is the string gas cosmology approach. But there could be more...

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Résonaances is a particle physics blog from Paris. It's about the latest news and gossips in particle physics and astrophysics. The main goal is to make you laugh; if it makes you think too, that's entirely on your own responsibility...